There are several arguments I can see in favor of a fork.
1. Ensure that there will be a systemd-free variant of debian even beyond jessie.
2. Provide a tangible metric of the interest in such a variant.
3. Provide a platform on which alternatives to systemd can be developed and promoted. After all, systemd it must be said, addresses some legitimate concerns, largely concerned with service tracking and a generally far more dynamic computing environment. The recent Future of FreeBSD presentation by Jordan Hubbard is actually one of the more eloquent arguments in this light (noting that FreeBSD is both Not Linux and Doesn't Have Systemd).
The primary argument against forking is that it divides efforts. My counterargument is that forking makes the existing divide between views on a given approach tangible and explicitly visible. Free Software, particularly under the GNU GPL or equivalent copyleft, also explicitly preserves the right to merge projects which have forked, as has in fact been the case in several notable instances -- GCC/EGCS, the long-running TK
For a specific study of the topic, see "A Comprehensive Study of Software Forks: Dates, Reasons and Outcomes" by Gregorio Robles and Jesuus M. Gonzalez-Barahona
http://flosshub.org/sites/flosshub.org/files/paper_0.pdf
It cites: GCC/EGCS, xpdf/poppler, GNU Emac/Xemacs, the numerous OpenOffice forks, X.org/XFree, NetBSD/OpenBSD, Wireshark/Ethereal, Gerequi/Amarok, MySQL/Miriadb. In total the paper explores some 220 forks, analyzing them by software area (e.g., networking, web apps, development, etc.), year of initiation, and reason. "Technical" accounts for the most at 27%, "differences among technical team" is sixth at 7.3%.
Resolution of forks is of particular interest:
Successful branching: 43.6%
Discontinuation of original: 29.8%
Discontinuation of fork: 13.8%
Discontinuation of both original and fork: 8.7%
Re-merging of fork: 3.2%
Other: 0.9%
As a critic of systemd and proponent of the concepts of Free Software, including the Freedom to Fork, I'm in support of this and similar efforts.